This invention relates to aluminum-treated steel products, i.e. non-rimming steels to which aluminum has been added for killing or grain refinement, or both, and which nevertheless are not seriously characterized by the surface defect problems often caused by the presence of aluminum at or near the surface of an ingot, bloom, slab or the like. As will be understood, such problems chiefly arise from inclusions constituted of aluminum oxides or nitrides, but in any case, the present invention reduces or avoids significant aluminum content in the skin of the ingot (which is wholly non-rimmed, both skin and core) while maintaining adequate concentration for desired purposes in the main body or core of steel and of products rolled from such ingot.
As noted above, the invention is chiefly concerned with steels of non-rimming character, particularly compositions outside the usual rimming range, as by inclusion of higher amounts of carbon or manganese, or both, as well as possibly other elements, e.g. silicon, nickel, chromium, which might also interfere with rimming action. In particular, having regard to the fact that steels basically cannot exhibit the effervescent rimming action when containing substantially more than or even as much as 0.15% carbon or substantially more than about 0.65% manganese, the invention is primarily related to non-rimming steel, and in one preferred aspect further contemplates that the base composition, in unkilled state as first melted, should be such, as to deoxidation, that formation of blow holes at or near the surface of the ingot is inhibited. In any case, the invention provides a new product and a relatively simple and effective way of making such product, to achieve the conditions and circumstances noted above, especially for steels to be processed into bar products, for which there is great concern in regard to ingot or bloom defects.
Prior art has been noted that describes the production of steel having an outer skin of rimmed steel and a core which is killed with aluminum. Such products have been attained by taking a melt of rimming steel, pouring each ingot mold partly full, allowing the metal to rim while a skin solidifies next to the mold wall, and thereafter completing the fill of the mold with the same molten steel, while adding aluminum. This final or back-filling step mixes into the molten core, the ultimate result being an ingot that has a skin of rimmed steel and a core of killed steel which otherwise has the same composition.
Some more recent inventions in this area have related to addition of other and special alloying materials, at the time of back-filling, so that the core becomes characterized not only by a killed state, but by greater strength, toughness or the like, as may be contributed by the alloying elements. In general, these other developments, wherein the core has been killed by aluminum addition, have contemplated the attainment of a rimmed steel skin surrounding the core, it being conceived that such type of skin is necessary in order to achieve significant avoidance of skin defects, and indeed such nature of the skin has been especially desirable where the steel was processed, by ultimate cold rolling and annealing, to achieve a true deep-drawing grade.
The foregoing prior or other developments have included not only disclosures in prior patents, but also some presently developed inventions which are the subject of other patent applications. Attention has also been given to U.S. Pat. No. 3,590,476, Bernard S. Levy et al., issued July 6, 1971, which describes a method, and resulting steel products, wherein addition of tellurium or equivalent element for increasing machinability, is injected into the molten core of a freshly poured ingot after a surrounding steel skin has solidified. This patent purports to deal with various kinds of steel, including those with too much manganese to rim, and contemplates one method in which the ingot mold is partly filled, then left to solidify the non-rimming skin; thereafter, filling is completed while adding the desired tellurium or equivalent. The sole purpose of the described method and constitution of the product is to avoid a surface defect (after hot rolling) caused by tellurium and known as "surface checking", said to be manifested by a large number of small surface cracks, which render the article commercially unacceptable. The only concern of the Levy et al. method and product is to avoid this surface checking on the hot rolled product, i.e. in the special situation of addition of tellurium, selenium or lead, which may cause the trouble; no other compositions or problems are considered or discussed.
The present invention is directed to different areas from the foregoing prior art, especially in making steel, for bar stock, which is basically non-rimming, while the process and product are nevertheless such as to involve relatively superior surface conditions of the ingot or the immediately rolled articles. The chief aim of the product is to achieve an aluminum-treated composition, i.e. to produce a fully aluminum-killed steel or an aluminum-grain refined steel, or both, while avoiding certain common defects resulting from the addition of aluminum. Moreover, the products of the invention, for example by reason of suitable compositional control, are preferably devoid of so-called blow holes, as sometimes undesirably occur at or near the surface or skin of a rimmed or otherwise non-deoxidized ingot.